CHICAGO — Leaders of more than 20 leading liberal groups supporting both Hillary Clinton and Bernie Sanders are set to release a scathing letter on Tuesday that serves as a progressive call to arms against Donald Trump, imploring Americans to undertake a unified set of concrete steps to oppose the Republican front-runner — from protesting to organizing large-scale voter turnout efforts.

“This is a five-alarm fire for our democracy. A hate-peddling bigot who openly incites violence is the likely presidential nominee of one of our nation’s two major parties. It is alarming and dangerous. Donald Trump’s candidacy is a threat to the America we love, and we must respond to him and what he is stoking as such,” reads the letter, which was obtained first by POLITICO. It is signed by top officials of Sanders-backing groups like MoveOn.org and Clinton-backing organizations including the Service Employees International Union.

Many of these groups have primarily focused on mobilizing their millions of members — and spending significant sums of money — for their preferred candidate in the Democratic primary thus far. The letter recognizes a significant shift in thinking among them, with leaders in the party’s left wing now setting their sights more squarely on Trump rather than the protracted battle between Clinton and Sanders. The signatures are acknowledgments from leaders of those organizations that the time has come to more seriously confront Trump — who could, they warn, actually win.

“Today we are calling for a massive nonviolent mobilization of working people, students, immigrants, children of immigrants, great-great-grandchildren of immigrants, people of color and white people, the unemployed and under-employed, people of faith, retirees, veterans, women, and men — anyone who opposes bigotry and hate and loves freedom and justice — to stand up to Trump’s bullying and bigotry,” the note continues.

Other high-profile signers of the letter — which was set to be released in the opening hours of voting on Tuesday as Ohio, Illinois, Missouri, North Carolina and Florida go to the polls — include Michael Brune, executive director of the Sierra Club; Ilyse Hogue, president of NARAL Pro-Choice America; Annie Leonard, executive director of Greenpeace USA; and May Boeve, executive director of 350 Action. The text encourages more groups to sign onto the effort.

Anticipating the possibility that the billionaire real estate developer essentially locks up the Republican nomination on Tuesday — and that he subsequently pivots to a more widely acceptable message — the letter lists multiple examples of Trump encouraging violence at his rallies. To fight back, the group proposes a three-pronged response ahead of November’s general election, regardless of the Democratic nominee.

First, it encourages nonviolent organizing, which the letter says could take the form of community rallies, large-scale marches, teach-ins and social media campaigns.

“If the Republican Party nominates Donald Trump for president — and the odds are it will — there’s every reason to believe he has a serious shot at winning the general election,” note the progressives involved in the effort.

“We cannot afford to underestimate him until it’s too late, as many Republicans now regret having done during the primaries. If we wait to see how things shake out to make our plans, it’ll be too late and November will come sooner than anyone thinks."